Various members of the IVA, VA and VIA transition elements in the periodic table have properties which make them very useful in practical applications. Titanium, for example, has excellent corrosion resistance. Furthermore, its alloys have a high strength to weight ratio which makes them useful in applications where weight is important, such as in aircraft construction. In addition, titanium has been used extensively to construct biomedical implants because of its substantial non-toxicity to humans and animals.
Although titanium and its alloys have many excellent properties, this metal has a tendency to gall to mating surfaces of itself and almost any material. This weakness is exhibited in several situations. Titanium fasteners tend to seize in the threads during tightening so they cannot be driven to their full extent or, once so, cannot be loosened by backing off without destroying the fastener. In similar fashion, valve seats and valve spool threads are rendered inoperative. Despite effective lubrication, mechanical drive systems, such as gear trains made from high strength titanium alloys, exhibit abnormal rates of wear so that critical clearances cannot be maintained and freedom of motion in start and stop conditions is lost. Because of their excellent strength to weight ratios and ability to withstand high temperatures, titanium component parts would be used in shaft bearings of the roller, ball and sliding types operating at high temperatures if a means could be provided to overcome their tendency to gall.
Titanium alloys have been used in implantible human joint devices for the hip and knee. Such medical applications involve articulation of polished titanium surfaces against ultra high molecular weight polyethylene. In a few such applications, abnormal wear and consequent need for revision surgery have occurred. It appears that in such instances, the passive film that normally protects titanium from corrosion is somehow ruptured. It is believed that reformation of the passive film is prevented by galling of the titanium surface as it slides against the polyethylene under pressure.
In the various applications where titanium could be or is already widely used, a hard surface with improved anti-galling characteristics would be valuable. Since titanium carbide is one of the hardest metallic materials, a surface coating of this material could provide these properties. Various attempts to form carbides of metals are reported in Coating of High-Temperature Materials, ch. 2, H. H. Hausner, ed., Plenum Press, New York, 1966. Much of this work carried out in the Soviet Union involves gaseous deposition and vacuum deposition methods to carburize the metals. These methods were often carried out in the presence of hydrogen or hydrogen containing compounds as noted, for example, in U.S. Pat. No. 2,892,743. It is well-known that absorbed hydrogen has a detrimental effect on the mechanical properties of the metals. Furthermore, such methods do not coat the metal uniformly and are not cost effective where more than a few microns of carbide coating are required.
We have now discovered a method for forming a carbide coating of the metals of Group IVA, VA and VIA of the periodic table, which imparts wear resistance and galling resistance to the surface of the metals. Carbide coatings at depths in excess of about 5 microns resists the penetration of sharp, pointed objects of hardened steel. Furthermore, the coating process can be adapted to give a uniform and substantially continuous coating on all surfaces of the metal object being coating.